Gate turn-off thyristors (GTOs) are semiconductor switching devices which can be turned on and off by supplying bidirectional current to a gate or control terminal of the device. In addition to a control or gate terminal, the GTO has an anode and a cathode terminal. Current into the gate terminal will forward bias the GTO and allow an anode-to-cathode current. By reversing polarity of the gate current so that an anode-to-gate current path is formed, the anode-to-cathode voltage reverses and the GTO turns off. Thus, GTOs require gate drive current of two polarities. One form of gate drive circuit is shown in W. McMurray U.S. Pat. No. 4,593,204 issued June 3, 1986 and assigned to the assignee of the present invention.
The unique ability of the GTO to be turned off or commutated by reversing current polarity at the gate terminal has made the device popular in power converter circuits such as inverters and choppers. For such use, however, there are drawbacks to using circuits heretofore devised for effectively commutating the GTO under fault current conditions. Such condition may occur, for example, in an inverter in which two or more GTOs become simultaneously conductive and short-circuit a power source. Attempts to protect GTO converter circuits under fault current conditions have generally required bulky and costly extra components along with fast fault detection and complex special action by gated drive circuits.